Support for using a custom http.Agent

The Elasticsearch options support a parameter 'agent' that value
is undefined (default), false (no http polling) or
a custom instance of http.Agent

For example one could use this parameter with the
agentkeepalive (https://github.com/TBEDP/agentkeepalive)
to attempt to keep the http connection opened
Merge pull request #75 from hmalphettes/httpAgent

Support for using a custom http.Agent